The present invention relates, in general, to the field of injection molding plastic articles.
Nothing in the following discussion of the state of the art is to be construed as an admission of prior art.
A high surface quality and good molding properties can be achieved by heating a molding tool during or after the cavity has been filled with plastic material and by cooling the molding tool to a temperature below the demolding temperature. One approach suggested heretofore is the arrangement of a medium passageway in one mold portion or both mold portions of the molding tool and using the medium passageway to alternatingly conduct liquid coolant and liquid heating medium. This procedure results, however, in long cycle times because of the large masses that need to be heated or cooled and is thus unable to meet current demands for maintaining process parameters, for example when microstructures are molded for manufacturing CDs, DVDs, writeable CDs and writeable DVDs and the like.
German patent publication DE 195 33 045 A1 describes a molding tool having a mold portion formed near the cavity surface in a single plane with heating channels and neighboring separate cooling channels, for alternating conduction of a heating medium or a coolant. As a result, the temperature of the cavity surface fluctuates between regions in immediate proximity of the cooling channels, on one hand, and in regions in immediate proximity of the heating channels, on the other hand. These temperature fluctuations continue even when the heating system is switched off during the cooling phase, as heat cannot dissipate fast enough to attain in the regions of the heating channels the same temperature during the cooling phase as in the regions of the cooling channels. The same shortcoming applies during heating action, when the cooling channel system is off. The temperature profile of the cavity surface thus becomes uneven, thereby adversely affecting the quality of the plastic article. These shortcomings are especially of concern, when molding microstructures of CD substrates and DVD substrates are involved. In addition, the need to shut down the heating system during the cooling action is another drawback of this conventional mold because it prolongs the time required to re-heat the cavity surface to the initial starting temperature at the beginning of the injection process, after the molded article has been removed.
It would therefore be desirable and advantageous to provide improvements in the field of injection molding to attain high surface quality and superior molding capability of a plastic article being molded, while the overall process has short cycle times.